The goal of this research is to characterize the cell interactions underlying positional specification of sensory and motor neurons and their targets during development of the avian head. Emphasis will be on studies of the trigeminal- jaw complex. Studies of cell movements and lineages as well as changes in cell behavior resulting from experimental alteration in the spatial relations between precursors and their environment will be performed. The first aim will define interactions leading to the formation of specific connections between trigeminal motor nerves and jaw muscles. Lineage tracing of neuroblasts and myoblasts will be performed. Experimental strategies involve use of retroviral markers, injection of fluorochrome tracers, and surgical transplantation of rhombomeres (motor neuron precursors) and somitomeres (muscle precursors) between quail and chick embryos. The second aim focuses on specification of trigeminal sensory neurons derived from neural crest and placodal cells, and tests the hypothesis that the latter are dependent upon contact with the former for spatial patterning.This will be tested by surgical manipulation of precursors, followed by a detailed analysis of the central and peripheral projections of both graft- and host- derived trigeminal sensory axons using antibodies specific to quail or chick axons. The third aim will examine two features of developing craniofacial skeletal muscles, the origins of primary and secondary myocytes and myocyte degeneration. The proposed studies will determine whether avian primary and secondary myocytes have common or separate embryonic origins. Most primary myocytes in developing head muscles degenerate between days 12-15 in the chick embryo. Studies are planned to study the relationships between innervation and myocyte degeneration during craniofacial muscle development. The possibility that changes in innervation patterns are related to myocyte degeneration will be studied. The final aim examines the fate of single neural crest progenitors to study the strategies used by neural crest cells to generate the diversity of phenotypes found in the head. Clonal analyses of neural crest cells will be performed in vivo with retroviral markers. These investigations are expected to define how neurogenic and myogenic cells move and interact before forming functionally integrated tissues in the developing head.